This invention relates to acoustical liners designed to absorb sound energy in the low frequency range and particularly adaptable for turbofan engines.
This invention can best be appreciated by referring to FIG. 1 exemplifying the prior art showing only two adjacent cavities included in an array of cavities and the tubes communicating the grazing flow internally thereof. A resistive element may be located at the inner end of the tube as shown or elsewhere in the tube and the cavities, tubes and resistive elements all being sized for maximum sound absorption for the application for which it is intended to be used. In this type of configuration the design of a liner for a particular frequency range is somewhat limited inasmuch as the optimum impedance value for only one frequency is attainable, such that the maximum energy absorption may not be achieved.
We have obviated this problem and obtained an improved liner with increased flexibility in the design of the liner configuration over a range of frequencies by acoustically coupling two or more adjacent cavities. The adjacent cavities are asymmetrical either by virtue of cavity arrangement, opening configuration or resistive material such that pumping air results in the interconnecting opening between adjacent cavities wherein additional dissipation of sound energy is evidenced. This provides for additional optimum impedance values at two or more frequencies, which when taken into consideration affords greater sound absorption and flexibility in the design of the liner. As a result this invention affords a reduction of the size of the suppressor device required to absorb a given amount of sound energy. Such a device is particularly important in a turbofan engine application for supressing low frequency sound where space and weight are critical parameters.